Implement Hierarchical Timing specification for all animators in accordance with Google Material Design motion guidelines

[aphexcx]

Goal

Adhere to the Google Material Design Hierarchical Timing spec, illustrated in this video.

Motivation

The Google Material Design spec for recycler view animations specifies:

When building a transition, consider the order and timing of element movement. Ensure that motion supports the information hierarchy, conveying what content is most important by creating a path for the eye to follow.
recyclerview-animators does not currently follow this specification - all elements animate in at once.

Pull Request

In this PR, I've implemented the Hierarchical Timing specification for all animators, achieving the cool effect that the spec video shows.

Here's a video of my implementation in action, in my app:
https://www.dropbox.com/s/gxiw63pn7ljv3if/recyclerview-animation.mp4?dl=0

Implementation details

I achieved this by adding a .setStartDelay call to each Animator's animateAddImpl and animateRemoveImpl method. They call two new methods on BaseAnimator: getAddDelay and getRemoveDelay respectively. These methods will determine the delay based on:

1. the element's position in the adapter
2. the current add or remove duration set by .setAddDuration or setRemoveDuration (currently, this is retrieved and then divided by 4 - I found this gives the best result and provides smooth, overlapping animations that are closest to the spec video.)

Let me know if you need more information to merge this. Thanks for the great library!

[wasabeef]

@aphexcx

Great!
Let me confirm.

[wasabeef]

@aphexcx
Thank you
2.2.0 released

[aphexcx]

💃